1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus such as a laser printer for performing an image formation by supplying toner to an electrostatic latent image formed on a circumferential surface of a photosensitive drum, developing the image, and transferring the developed image onto a paper sheet. In particular, the present invention relates to a developing device capable of applying bias voltage to a supply roller for supplying toner onto a developing roller, whereby charged toner is transferred to the developing roller.
The present invention also relates to a developing device for an image forming apparatus capable of easily controlling a direct-current (DC) power device for a constant-current, the DC power device being connected to a toner supply roller, regardless of variation of a resistance value between a developing roller and the supply roller, to prevent breakage of the DC power device even when it stops, thereby to form a visual image with a good image quality.
Furthermore, the present invention relates to a developing device for an image forming apparatus capable of effectively preventing poorly charged toner which is produced in a toner storing unit after toner supply from adhering to the developing roller even when a surface deterioration of the developing roller caused due to aging and an increase of deteriorated toner remaining in a developing chamber, thereby to form a visual image with a good image quality.
2. Description of Related Art
Heretofore, there have been proposed various types of developing device for an image forming apparatus, e.g., a laser printer and the like. The developing device is generally constituted of a toner storing unit including a toner cartridge, an auger member for dispersing the toner supplied from the toner storing unit into a developing chamber, a toner supply roller for supplying the dispersed toner, a developing roller for performing toner development by providing the toner supplied by the toner supply roller onto an electrostatic latent image formed on a circumferential surface of a photosensitive drum. One example of a toner supply device as the developing device is shown in FIG. 11. FIG. 11 is an explanatory view showing a schematic sectional view of a main part of the developing device for an image forming apparatus in the prior art.
In FIG. 11, the toner supply device has a toner cartridge 100 which accommodates therein toner. The toner cartridge 100 is provided with a toner exhaust port at an axial center portion of the cartridge 100. In the toner cartridge 100, disposed is an agitator 103 for agitating toner to supply it through the toner exhaust port to a developing chamber 102 side. In a frame F, a toner introduction port is formed corresponding to the toner exhaust port of the toner cartridge 100, both ports constituting a toner supply port 101. The frame F includes an upper frame F1 and a lower frame F2 which form a closed developing chamber 102. In this developing chamber 102, two auger members 109 and 110 are rotatably disposed near the toner supply port 101 (at a right side in FIG. 5), to uniformly spread the toner supplied through the toner supply port 101 in the developing chamber 102.
At a lower side in the developing chamber 102, a toner supply roller 104 is rotatably disposed and a developing roller 105 is rotatably arranged facing the supply roller 104. This toner supply roller 104 serves to supply the toner transported through the toner supply port 101 to the developing roller 105. Here, the toner supply roller 104 comprises a roller shaft 104A made of metal (steel materials of various kinds) and a conductive sponge member 104B for surrounding a periphery of the roller shaft 104A. The developing roller 105 comprises a roller shaft 105A made of metal (steel materials of various kinds) and a conductive rubber member 105B which is generally harder than the conductive rubber member 104B and is provided surrounding the roller shaft 105A. At the time of toner supply from the toner supply roller 104 to the developing roller 105, the toner supply roller 104 is in contact with the developing roller 105 such that the conductive sponge member 104B is somewhat compressed to the conductive rubber member 105B.
To the roller shaft 105A of the developing roller 105, a constant bias voltage is applied from a constant voltage DC power device 112. To the roller shaft 104A of the toner supply roller 104, similarly, a constant bias voltage is applied from a constant voltage DC power device 111 so that the charged toner in the toner cartridge 100 is made to move from the toner supply roller 104 to the developing roller 105.
Such the application of bias voltage to the supply roller 104 can cause active transfer of the toner having a good electrification property onto the developing roller 105. Even when a larger amount of fresh toner which is not charged satisfactorily is supplied to the developing chamber 102. Accordingly, it is possible to prevent selective collection and supply of only the insufficiently charged toner with respect to the developing roller 105, and the occurrence of fogging which may be caused by the toner supply of a large amount of the insufficiently charged toner to the developing roller 105.
A blade 107 is secured on an inner wall of the upper frame F1, above the developing roller 105, by a fixing member 106. This blade 107 serves to regulate the thickness of a toner layer formed on a circumferential outer surface of the developing roller 105. The developing roller 105 is disposed so as to be in contact with a photosensitive drum 108. An electrostatic latent image can be formed on a circumferential outer surface of the photosensitive drum 108 by a light exposure unit (not shown) which scans the surface of the drum 108 with laser light in accordance with image data. The developing roller 105 then performs toner development by supplying toner on the electrostatic latent image formed on the surface of the photosensitive drum 108. The developed image is transferred onto a paper sheet fed from a paper supply unit (not shown) to print an image thereon.
However, in the conventional developing device for an image forming apparatus, as an operation time thereof becomes longer, insulating materials, e.g., silica, etc. covering toner particles tend to adhere to respective surfaces of the conductive sponge member 104B of the toner supply roller 104 and the conductive rubber member 105 of the developing roller 105, thus forming an insulating film on the surfaces of the members 104 and 105. The bias voltage are to be applied from the constant voltage DC power device 111 and 112 to the toner supply roller 104 and the developing roller 105 respectively, so that a difference between those bias voltages is constant. The insulating film causes the current passing between the toner supply roller 104 and the developing roller 105 to become minute (for instance, 0.1 .mu.A or less) or zero, thus resulting in a problem of preventing the transfer of the toner positively charged by an electrical field from the supply roller 104 to the developing roller 105.
If a sufficient amount of toner is not carried on the conductive sponge member 104B, the resistance between the supply roller 104 and the developing roller 105 decreases (for instance, 2 M.OMEGA. or less), allowing an overcurrent (e.g., 50 .mu.A or more) to flow therebetween. This can result in overcharging of toner carried on the sponge member 104B or local accumulation of toner negatively charged.
Even when the constant voltage DC power device 111 for applying bias voltage to the toner supply roller 104 is exchanged to a DC power device for a constant-current control, there is a problem that the output voltage becomes less than the voltage that the DC device can control if the resistance (for instance, 2 M.OMEGA. or less) between the supply roller 104 and the developing roller 105 decreases.
When the DC device for applying bias voltage to the supply roller 104 is turned off, the electric potential of the DC device becomes 0V, allowing reverse current to flow between the supply roller 104 and the developing roller 105. This results in transfer of the reversely charged toner to the developing roller 105.
The bias voltage with respect to the toner supply roller 104 is constantly applied from the constant-voltage DC power device 111, which always consumes electric power.
Furthermore, constant application of bias voltage to the supply roller 104 enables an increase of sufficiently charged toner, resulting in no chance of supplying a large amount of insufficiently charged toner to the developing roller 105; however, only the toner which has a good electrification property is supplied to the developing roller 105, and also the toner which has a poor electrification property is not consumed and such the toner will be accumulated in the developing chamber 102.